HELP!!! Tenant(s) keeps leaving front communal door open

[Brandon Styles]

Hello BP'ers! 

I have a problem.  I have a tenant in my 4 unit apartment building that's leaving the front communal door open. Whoever is doing it is leaving the front door mat in between the front door. 

What can I do to stop this from happening? I want to leave a note/notice in the hallway and give each tenant a letter addressing the issue but I don't know what/how to say it. Also what else can/should I do besides writing a notice? Should I put up a security camera to show that I'm serious? Should I put some sort of alarm on the front door that goes off after x amount of minutes? Any and all advice is greatly appreciated! 

[Michael Boyer]

Brandon, I like your tenant communication angle, and maybe you could ask for their "help" first and focus on how it is in their interests (safety, energy efficiency, keeping costs (and rent) low)... Also, it helps you manage the place by excluding people who should not be there etc. Perhaps empower them all to close it, keep it closed and even check if they can. And if this solution does not work, let them know you will explore monitoring, fines, etc. That would be my first step, then I may explore discovering who is the  culprits(s), technologies or other solutions...best of luck